This invention relates to certain kinds of thermal processes and more particularly to a novel system and method for the efficient execution of such.
There are several types of processes to which the present system is applicable. Examples of such include treatment of metal sulfide ores for removal of sulfur and combustion of organic materials, such as sewage sludges, for heat production. For example, some metal ores of commercial significance, such as ores of copper, lead and zinc, occur in nature primarily as sulfides. An initial step in the recovery of the metal in such ores is a roasting step which can be accomplished using the subject system in which the metal sulfide is thermally decomposed, the sulfur being removed in the form of sulfur dioxide. Also, waste organic material, such as sawdust and other similar by-products can be utilized for their fuel value by pyrolysis. In such a process carried out by the subject system, combustible gases are driven off by thermal decomposition. In addition, the subject system may be used in the treatment of sewage sludges in which the water is to be removed and the organic content reduced by incineration. These processes are cited as examples only and this recitation is not intended to limit the potential scope of application of the invention.
A common feature of all potential applications is that they either require a substantial energy input or, where exothermic processes are involved, require close and continuous temperature control. Conventional systems and apparatus, such as rotary kilns, multiple hearth furnaces, and fluidized bed incincerators, are generally designed to effect this energy input or control primarily through thermal conduction, convention, or diffusion. A description of the mode of operation of these conventional devices will serve to describe the essential differences between such devices and the present invention.
A rotary kiln is a device in which a long cylindrical chamber, horizontally disposed, is made to rotate about its axis. The inner surface of the cylinder is fitted with radial vanes. Solid matter placed into the cylinder is tumbled and mixed by the action of the vanes. By tilting the axis of the cylinder, the solid matter is also caused to progress from one end of the cylinder to the other as it is tumbled. By flowing hot gases through the cylinder at the same time, energy may be transferred from the hot gases to the solid matter.
A multiple hearth furnace is comprised of a series of circular hearths arranged one above the other. The solid matter is made to traverse the furnace from top to bottom, transferring from hearth to hearth while the hot gases flow from the bottom to the top. Each hearth is provided with a radial slot and a radial bar, called a rabble arm, pivoted at the end which is located at the center of its hearth and caused to sweep around the hearth slowly. The material is deposited on the top hearth where it is exposed to the hot gases while it is slowly raked around the hearth by the rabble arm until it reaches the radial slot whereupon it falls through the slot to the hearth below. This process is repeated as many times as there are hearths. The combination of raking by the rabble arms and the process of transferring from hearth to hearth in the hot furnace area progressively exposes more and more of the solid matter to the hot atmosphere until the desired condition is attained.
A fluidized bed type or incinerator operates on a somewhat different principle. The desired high temperature atmosphere is created by heating a bed of fine sand to the desired temperature. This bed is caused to have the properties of a liquid by the buoyant forces developed by hot gases flowing upwardly through it, sometimes in conjunction with a mechanical vibrating force applied to the bottom of it. The solid matter to be processed, which must be in a finely divided form as presented to the bed, is injected into the bed, whereupon the mechanical forces of the moving sand particles cause it to be further fragmented and dispersed.
Despite the availability of the above equipment, there remains a need for a system in which such thermal processes can be carried out quickly, efficiently and with relatively minimal equipment outlays. It is accordingly a primary object of the present invention to provide a system which accomplishes the above objectives in a novel and continual manner.
These and other objects of the present invention are accomplished in part by the use of mechanical energy in an appropriate way to reduce the solid or liquid matter to a finely divided state, either prior to or coincident with the introduction of such matter to the carrier gas in order to create as large a surface area of contact as possible. The second essential part is that there be set up a reaction chamber through which the carrier gas stream and the finely divided solid or liquid matter may be caused to pass and such that carrier gas is caused to flow at such velocities that substantially all of the solid or liquid matter becomes entrained therein and such that a substantial fraction of the carrier gas is caused to pass through the reaction chamber two or more times before leaving the chamber system.
In order to provide desired interaction between the solid or liquid and the carrier gas, the system is virtually dependent on substantially complete entrainment of the solid or liquid and recirculation of a substantial portion of the material laden gas within the system. This is necessary to provide the desired boundary layer interaction between the material and the gas and also to provide the desired residence time of the material and the gas within the reactor system. As a result of a combination of these effects the system herein disclosed is operative for effecting substantially complete interaction between the liquid or solid particles and the carrier gas.
The device of the instant invention incorporates means for effecting the maximum possible rate of interaction without unnecessarily raising temperature utilizing two factors; (1) maximization of the specific surface area (surface to volume ratio) for the solid or liquid substance and (2) rapid renewal of the boundary layer at the interface between the gaseous and the solid or liquid phases. By optimizing these factors the instant invention provides an interaction system which is substantially more energy efficient than those systems previously known.
Other types of rate-enhancing results are achieved by the device of the instant invention. For instance, even though evaporation, which is an endothermic process, is rate-controlled primarily by temperature, it is also dependent upon the level of the vapor pressure in the boundary layer environment. Were the boundary layer stagnant, the vapor pressure would quickly build up to that level where no further evaporation could occur at any temperature. The more rapid the renewal of the boundary layer, the higher can be the difference in vapor pressure between the environment and the surface of the substance from which matter is being evaporated.
In the chemical reaction type of process, such as the oxidation of sulfide ores or the organic function of those waste water treatment sludges which contain organic material, the rate of reaction depends upon, in addition to temperature, the concentration of oxygen molecules in the boundary layer. Again, the more rapid the renewal of the boundary layer, the more rapid the reaction rate at any given temperature above the ignition level.
Further, improvements in efficient energy utilization are achieved by operating without refractory materials and limiting operation to temperatures within the limitations of certain special alloy steels and by utilizing reaction chambers of substantially reduced volumes.
A necessary element of such a system is that the carrier gas be driven through the reaction chamber by a fan or blower, capable of operating at the necessary temperatures and having sufficient power to set up and maintain the necessary gas velocities.
Alternate ways of physically creating the stated conditions may be suggested as illustrative of the application of the present invention.
In one proposed embodiment, the reaction chamber may be a vertical cylinder, the diameter of which is chosen such that the upward velocity of the carrier gas in the direction of the cylinder axis when introduced at the base of the cylinder is small enough that substantially all of the finely divided solid or liquid matter introduced at the top of the cylinder will be allowed to fall through the chamber under the influence of gravitational forces. The necessary entrainment conditions are created by causing the fan or blower to inject the carrier gas at the base of the cylinder tangential to the cylinder walls and at such velocity that a vortex is created in the reaction chamber in which the tangential component of velocity is anywhere from ten to twenty times the vertical component of velocity. The necessary recirculation condition is achieved by directing the exiting gases from the top of the reaction chamber to the suction intake of the fan or blower and equipping the output pressure discharge of the blower with an adjustable flow splitter, with part of the discharge being directed to exhaust from the system while the balance is directed to the input nozzles at the base of the reaction chamber.
Alternatively, the same objective may be achieved by utilizing a jet mill of particular construction as a reaction chamber. Such device has a substantially closed wall toroidal body and means for continually recycling a high temperature carrier gas stream therein such that as the material to be thermally processed is fed thereto, the gas stream reduces the solid particles thereof into a size range of particles which become entrained therein, and as the particles and the gas stream come in contact with each other, the desired contact and transfer of thermal energy occur. The toroidal flow path of the gas stream enables a portion thereof traveling about the inner confines of the toroidal body to be continually withdrawn from the air stream by tapping an exit port into the side wall of the jet mill. In this embodiment, the necessary carrier gas velocities are established by a pressure blower which injects such gas tangentially to the wall of the toroidal structure through nozzles which are sized to impart the desired velocities. The necessary recirculation condition is made to occur within the structure itself by controlling the geometry of the exit port.
Other objects, features and advantages of the invention shall become apparent as the description thereof proceeds when considered in connection with the accompanying illustrative drawings.